Vanity Light
Do you have the wiring harness for the vanity light?
If so, remove the entire back seat set-up (top and bottom) as well as the quarter trim panel on the side you want to install the light. Both tasks are easy.
Then remove the small, rectangular-shaped wood-grain insert where the light is supposed to go. Install the vanity light/housing into the opening in the quarter trim panel and run the wire down underneath the panel into the cabin area. If memory serves, there are a couple of screws involved in the installation of the vanity light housing. To do this stage correctly, you'll need to have the quarter trim panel outside of the car. Then, once the light has been attached, re-install the quarter trim panel into the cabin.
The wiring harness will run on the floor of the car with other wires and then plug into a brown, female socket on the driver side of the trunk area, up and behind where the spare tire sits. All T-Birds were supposed to come with this (accessory) female plug. Harness and socket should be a perfect fit. Once plugged in, the light will come on when you open
either door.
Once you see it work, re-install your back seat. Not complicated at all. Hope you can find a set of vanity lights. Looks real classy when done! Have fun!